What happened?

I don't see this as a problem for Eldar. Newcomers are newcomers and Eldar simply are the most sensitive faction to when teamplay doesn't work. You feel it much stronger than on the other factions because the individual player is less capable of carrying.

Newcomers will learn and become good or they'll move on. The Eldar vets will still be here by that point and reckoning will be had. Until then, let the other factions enjoy their hollow "Eldar OP and still you lose! omg, youz so bad..." gloating. Grin, and let them dig their holes, it's the Eldar way of things.

In the meantime, don't spare the rod. Voice what is going wrong, it's how I learned the ropes and it's how those newcomers will fall in line too in time.